Stabilizing compositions for lubricants

ABSTRACT

A lubricant composition is disclosed that comprises lubricating oil and a mixture of at least two antioxidants, the first antioxidant being a substituted diphenylamine and/or a heterocyclic amine and the second antioxidant being a substituted phenol. Also disclosed is a method of increasing the oxidation stability of a lubricating oil comprising adding thereto at least two antioxidants, the first antioxidant being a substituted diphenylamine and/or a heterocyclic amine and the second antioxidant being a substituted phenol.

I claim the benefit under Title 35, United States Code, §119 to U.S.Provisional Application No. 60/776,934, filed Feb. 28, 2006 entitledSTABILIZING COMPOSITION FOR LUBRICANTS.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to improving the oxidation stability oflubricants, especially hydrocarbon based lubricating oils, by addingthereto a combination of at least two antioxidants.

2. Description of Related Art

Lubricants, such as those used in a variety of machinery, aresusceptible to oxidative deterioration during storage, transportation,and usage, particularly when such lubricants are exposed to hightemperatures and iron catalytic environments, which greatly promotetheir oxidation. This oxidation, if not controlled, contributes to theformation of corrosive acidic products, sludge, varnishes, resins, andother oil-insoluble products, and may lead to a loss of designatedphysical and tribological properties of the lubricants. It is thereforea common practice to include an antioxidant in lubricants to prevent, atleast to some extent, oxidation, so as to extend their useful life.Lubricant compositions containing various diarylamines or phenoliccompounds as antioxidants are widely known in the art.

U.S. Pat. No. 2,718,501 discloses a stabilizer system consisting of anaromatic amine with at least two aromatic rings, includingpara-phenylenediamine, and an organic aliphatic sulfur compound, whichis said to be suitable for stabilizing mineral hydrocarbon lubricatingoils, synthetic hydrocarbon oils, and polyalkylene glycol oils.

U.S. Pat. No. 5,091,099 discloses a phosphite-free lubricating oilcomposition which comprises a mineral oil or a synthetic oil or amixture thereof, and a mixture containing at least one aromatic amineand at least one phenol. The lubricating oil compositions are said to behighly resistant to aging and are said to be effective in preventingblack sludge formation.

U.S. Pat. No. 5,229,442 discloses stabilizing compositions for organicpolymers, composed of mixtures of at least one liquid phenolicantioxidant and at least one aminic antioxidant, subjected to thermaltreatment, that are characterized by their stabilizing capacity, whichis considerably higher than that of either single components or ofcorresponding untreated mixtures. These stabilizing compositions can beused in all fields where the single components of the mixtures aregenerally used and, preferably, in the stabilization of organic polymersnormally subject to deterioration due to oxidation processes.

U.S. Pat. No. 5,523,007 discloses a lubrication system comprising adiesel engine lubricating oil which is stabilized with an ester of asterically hindered phenolic compound and the use of an ester ofthio-ester compound for stabilizing diesel engine lubricating oils.

WO 94/22988 discloses a fuel composition said to improve the antiwearand viscosity controlling properties of an internal combustion enginelubricating oil during operation of the engine. Small amounts of thefuel composition combine with the engine lubricating oil during engineoperation and this provides an antioxidant boost to the lubricating oil.Preferably the fuel contains at least 57 g/1000 liters of a substituteddicyclic aromatic amine which is free of benzylic hydrogen atom s suchas a mono- and/or di-α-methyl styrene alkylated phenylenediamine and/ora hindered phenol such as a monostyrenated mono-isobutenated cresol ordi C₁₆ alkyl phenol. A synergistic effect is said to be demonstrated bya mixture of the aromatic amine and hindered phenol.

JP 53,051,206 discloses N,N′-2-naphthyl-p-phenylenediamine as anantioxidant to improve the oxidation stabilities of ester or mineral oilbased lubricating oils that also contain disulfides.

JP 59,020,392 discloses a lubricant composition comprisingN,N′-di-sec-butyl-p-phenylenediamine for pressure forming of oil tanks.The lubricant composition also contains hindered phenolic antioxidant.

The foregoing disclosures are incorporated herein by reference in theirentirety.

SUMMARY OF THE INVENTION

It has now been discovered that certain phenolic/aminic antioxidantblends offer unique synergy and improved performance, in terms ofanti-oxidancy, when used within a lubricant base stock and/or lubricantformulation.

By “antioxidant blend” is meant a blend for treatment of industrial andautomotive lubricants, which comprises a combination of aminic andphenolic antioxidant additives, and all plausible post-add and/orpost-heat by-products thereof that may occur owing to action/reaction ofsaid antioxidants via pre-blending and/or pre-heating of saidantioxidants, and/or to action/reaction of said antioxidants within alubricant base stock and/or lubricant formulation.

By “lubricant base stock” is meant a lubricant, such as mineral andsynthetic base oils chosen from Group I, Group II, Group III, Group IV,as well as poly-alpha-olefins (PAO) and natural and synthetic esters,including polyol esters.

Base oils in Groups I-V are broadly specified in the American PetroleumInstitute (API) Base Oil Interchangeability Guidelines. The five baseoil groups are described in Table 1.

TABLE 1 Sulfur API Base Oil Category (%) Saturates (%) Viscosity IndexGroup II ≦0.03 and ≧90 80 to 120 Group III ≦0.03 and ≧90 ≧120 Group IVAll polyalphaolefins (PAOs) Group V All others not included in Groups I,II, III or IVTypically, base stocks are the above types of lubricants with onlyminimal additional additives, including, but not limited toantioxidant(s), rust inhibitor(s), metal passivator(s) and the like.

By “synthetic esters” is meant esters based on reaction products ofpolyols and carboxylic acids. Examples of typical polyols used to makesynthetic esters include, but are not limited to, neopentyl glycol,trimethylolpropane, pentaerythritol, and dipentaerythritol, which arereacted with carboxylic acids, for example, valeric acid, isopentanoicacid, hexanoic acid, heptanoic acid, octanoic acid, and the like.Examples of commercial synthetic esters include, but are not limited to,polyol esters, trimellitates, adipates, oleates, and the like.

By “lubricant formulation” is meant a lubricant base stock, as describedabove, plus additional additives, including, but not limited to,antioxidant(s), rust inhibitor(s), metal passivator(s), ashlessdispersant(s), anti-wear additives, extreme pressure (e.p.) additivesdetergents, and the like, and by-products thereof.

By “by-products thereof” is meant any by-product, reaction product,decomposition product, and the like that would be potentially,theoretically, and/or feasibly formed from the chemical and/orthermal/heat action/reaction between these components with and withinthe lubricant formulation.

More particularly, the present invention is directed to a lubricantcomposition comprising:

(A) at least one lubricating oil selected from the group consisting ofnatural and synthetic lubricating base oils;

(B) at least one first antioxidant selected from the group consistingof:

-   -   (1) diphenylamines represented by the formula

wherein R₁ and R₂ are independently selected from the group consistingof hydrogen, alkyl, styryl, and α-alkyl styryl, provided that at leastone of R₁ and R₂ is not hydrogen; and

-   -   (2) heterocyclic amines of the formula

wherein n is an integer of from 0 to 50; and

(C) at least one second antioxidant selected from the group consistingof phenols represented by the formula

wherein R₃, R₄, R₅, and R₆ are independently selected from the groupconsisting of alkyl moieties and X is selected from the group consistingof sulfur, substituted or unsubstituted nitrogen, oxygen, alkylene,alkylene-S-alkylene, alkylene-O-alkylene, and —S—S—.

In another aspect, the present invention is directed to a method ofincreasing the oxidation stability of a lubricant comprising addingthereto

(A) at least one first antioxidant selected from the group consistingof:

-   -   (1) diphenylamines represented by the formula

-   -    wherein R₁ and R₂ are independently selected from the group        consisting of hydrogen, alkyl, styryl, and α-alkyl styryl,        provided that at least one of R₁ and R₂ is not hydrogen; and    -   (2) heterocyclic amines of the formula

-   -    wherein n is an integer of from 0 to 50; and

(B) at least one second antioxidant selected from the group consistingof phenols represented by the formula

wherein R₃, R₄, R₅, and R₆ are independently selected from the groupconsisting of alkyl moieties and X is selected from the group consistingof sulfur, substituted or unsubstituted nitrogen, oxygen, alkylene,alkylene-S-alkylene, alkylene-O-alkylene, and —S—S—.

In another aspect, the present invention is directed to a method ofincreasing the oxidation stability of a lubricant comprising addingthereto

-   -   (1) diphenylamines represented by the formula

-   -    wherein R₁ and R₂ are independently selected from the group        consisting of hydrogen, alkyl, styryl, and α-alkyl styryl,        provided that at least one of R₁ and R₂ is not hydrogen; and    -   (2) heterocyclic amines of the formula

-   -    wherein n is an integer of from 0 to 50; and

(B) at least one second antioxidant selected from the group consistingof phenols represented by the formula

wherein R₃, R₄, R₅, and R₆ are independently selected from the groupconsisting of alkyl moieties and X is selected from the group consistingof sulfur, substituted or unsubstituted nitrogen, oxygen, alkylene,alkylene-S-alkylene, alkylene-O-alkylene, and —S—S—.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As stated above, the diphenylamines used as the first antioxidant in thepractice of the present invention are represented by the formula

wherein R₁ and R₂ are independently selected from the group consistingof hydrogen, alkyl, styryl, and α-alkyl styryl, provided that at leastone of R₁ and R₂ is not hydrogen.

Where R₁ and/or R₂ are alkyl, they are preferably independently selectedalkyl groups of from 1 to 12 carbon atoms, which may be branched orstraight-chain, e.g., methyl, ethyl, propyl, butyl, pentyl, hexyl,heptyl, octyl, nonyl, decyl, undecyl, dodecyl, isomers of the foregoing,e.g., t-butyl, 2-ethylhexyl, and the like, and mixtures of theforegoing.

Where R₁ and/or R₂ are α-alkyl styryl, the alkyl group is preferablylower alkyl, e.g., alkyl of from one to four carbon atoms, such asmethyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, isobutyl, ort-butyl. Methyl or ethyl are more preferred, methyl most preferred.

Those skilled in the art will realize that the diphenylamine cancomprise mixtures of alkylated/styrenated and non-alkylated/styrenatedcomponents and that these components could theoretically includenon-alkylated and mono-, di- and tri-alkylated variants within thealkylated diphenylamine mixture.

Such alkylated and/or styrenated diphenylamine additives can be blendedwith or substituted by the heterocyclic amine antioxidant of the formula

wherein the number of repeating units, n, is an integer of from 0 to 50.Commercially available examples of such compounds include Naugalube (NL)TMQ, Durad AX51, and Durad AX53.

A second antioxidant of the present invention is a phenol represented bythe formula

wherein R₃, R₄, R₅, and R₆ are independently selected from the groupconsisting of alkyl moieties and X is selected from the group consistingof sulfur (—S—), substituted or unsubstituted nitrogen, oxygen,alkylene, alkylene-S-alkylene, alkylene-O-alkylene, and —S—S—. By“alkylene” is meant a hydrocarbon of the formula —C_(n)H_(2n)—.Preferably n is an integer of from 1 to 10, e.g., methylene, ethylene,propylene, butylene, pentylene, hexylene, heptylene, octylene, nonylene,decylene, isomers of the foregoing, and mixtures thereof. X ispreferably sulfur. The alkyl moieties attached to the phenolic rings arepreferably of from one to ten carbon atoms, e.g., methyl, ethyl, propyl,butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl and isomers of theforegoing, such as t-butyl and 2-ethylhexyl.

More preferably, the first antioxidant of the present invention is analkylated diphenylamine, where R₁ and R₂ are alkyl groups with thegeneral formula of C_(n)H_(2n+1), wherein n=4 to 9, suitably anoctylated/butylated and/or nonylated diphenylamine, and the secondantioxidant is a sulfur-containing alkylated phenol, wherein R₃ and R₄are alkyl groups with the general formula C_(n)H_(2n+1), wherein n=1 to4. Commercially available examples of such alkylated diphenylaminesinclude Durad AX57 (n=4 to 8), Durad AX59 (n=9), Naugalube 438,Naugalube 438L, and Naugalube 640.

Most preferably, the sulfur-containing alkylated phenol component of theabove blends would be of the formula

or the formula

Commercially available examples of such alkylated phenols include DuradAX16, Lowinox TBM6, Lowinox TBP6, and Durad AX18.

Generally, a lubricant base stock and/or lubricant formulation can betreated directly with the above-described aminic and phenolicantioxidant components.

Preferably, the aminic and phenolic antioxidant additives arepre-blended to form a homogeneous liquid mixture, which in turn is addedto the lubricant base stock and/or the lubricant formulation. Thehomogeneous liquid pre-blend mixture will preferably be fully miscibleand stable to precipitation of solids upon storage and handling for aminimum duration of time, suitably up to 24 hours to allow for ease ofhandling.

Typically, the homogeneous liquid mixtures are pre-blended at elevatedtemperatures of from about 50 to about 100 C for from about 0.5 to about6 hours, which will facilitate blending of viscous liquids and/or affordimproved performance in terms of anti-oxidancy.

It is known in the art to pre-blend aminic antioxidants of the typedescribed above with phenolic antioxidants of the structure

wherein R is C_(n)H_(2n+1) and n 6 to 20, to afford enhanced stabilizingcapacity for lubricant base stocks (see U.S. Pat. No. 5,229,442).

Preferably, the aminic and phenolic antioxidant additives can be blendedwith additional component(s) to form a homogeneous and fully miscibleliquid blends that are stable for longer periods of time, preferablyfrom 24 to 168 hours, more preferably for 168 to 336 hours, and mostpreferably for extended periods of time from 336 to 1000 hours or more.

Preferably, such additional component(s) can include, but are notlimited to, Group I, Group II, Group III, or Group IV base stocks,triaryl and/or tri-alkyl phosphate esters, alkyl mono- and di-acidphosphates, alkyl and/or aryl phosphites, liquid phenolic antioxidants,aminic antioxidants, carboxylate esters, aromatic hydrocarbons, andliquid glycols (such as, for example, ethylene glycol).

More preferably, the aforementioned additional component(s) are selectedfrom the group consisting of carboxylate esters, for example,trimellitate, adipate, phthalate, and oleate esters, and/or phosphateesters, such as trialkyl and/or triaryl phosphates and/or mixedalkyl/aryl phosphates.

Most preferably, the aforementioned additional component(s) are selectedfrom the group consisting of tri-isobutyl-phosphate (TiBP),tri-butoxyethylphosphate (TBEP), trioctyl phosphate (TOP), and alkylatedtriphenyl phosphate esters, such as butylated and/or propylatedtriphenylphosphate, and mixtures of the foregoing.

Lubricant base stocks that can have their antioxidant propertiesincreased by the antioxidant additive blend of the present inventioninclude a lubricant, such as mineral and synthetic base oils chosen fromGroup I, Group II, Group III, Group IV, as well as PAO (poly-α-olefins),natural and synthetic esters, including polyol esters, moreparticularly, but not limited to, circulatory oils, engine oils, gearoils, greases, hydraulic fluids, turbine fluids, and metal workingfluids.

Lubricant base stocks and/or lubricant formulations can have theiranti-oxidancy properties vary over a wide range. This can be achievedby:

-   -   1. Proper choice of aminic and/or phenolic antioxidants that        provide for best performance either individually or in        combination, in terms of anti-oxidancy, within the lubricant        base stock and/or lubricant formulation.    -   2. Changing the ratio of the antioxidant additives, such as the        ratio of the phenolic antioxidant additive(s) to the aminic        antioxidant additive(s), or    -   3. Keeping the aforementioned ratio constant while altering,        either increasing or decreasing, the treat rate of the        antioxidant additives in the base stock.

In other words, the components, either individually or in combination,can be tailored to provide the required performance characteristics, interms of anti-oxidancy, in a particular base stock and/or lubricantformulation.

In the preparation of the lubricating oil compositions of the presentinvention, the first and second antioxidants can be blended in thecompositions in a range of from about 0.01 to about 10 weight percenteach, preferably from about 0.1 to about 5 weight percent. Optionalcomponents can also be blended in the lubricating oil compositions in arange of from about 0.01 to about 10 total weight percent, preferablyfrom about 0.1 to about 5 weight percent. The content ratio of the firstantioxidant to the second antioxidant employed in the lubricating oilcompositions of the present invention can be in practically allproportions. But, preferably, the ratio will be in the range of 1:99 to99:1 parts by weight, more preferably, 90:10 to 10:90 parts by weight.

As noted above, the antioxidant mixtures of the present invention can beused in combination with other additives typically found in lubricatingoils, as well as other antioxidants. The additives typically found inlubricating oils are, for example, dispersants, detergents, antiwearagents, antioxidants, friction modifiers, seal swell agents,demulsifiers, VI (viscosity index) improvers, pour point depressants,antifoamants, corrosion inhibitors, and metal deactivators. Suchadditives are well known to those skilled in the art and there is noparticular restriction on the type of these additives for thisinvention. U.S. Pat. No. 5,498,809, incorporated herein by reference,discloses useful lubricating oil composition additives.

Examples of dispersants include polyisobutylene succinimides,polyisobutylene succinate esters, Mannich Base ashless dispersants, andthe like. Examples of detergents include metallic and ashless alkylphenates, metallic and ashless sulfurized alkyl phenates, metallic andashless alkyl sulfonates, metallic and ashless alkyl salicylates,metallic and ashless saligenin derivatives, and the like.

Examples of antioxidants that can be used in combination with theantioxidant mixtures of the present invention include dimethylquinolines, trimethyldihydroquinolines and oligomeric compositionsderived therefrom, thiopropionates, metallic dithiocarbamates, oilsoluble copper compounds, and the like. Examples of anti-wear additivesthat can be used in combination with the additives of the presentinvention include organoborates, organophosphites, organophosphates,organic sulfur-containing compounds, sulfurized olefins, sulfurizedfatty acid derivatives (esters), chlorinated paraffins, zincdialkyldithiophosphates, zinc diaryldithiophosphates,dialkyldithiophosphate esters, diaryl dithiophosphate esters,phosphosulfurized hydrocarbons, and the like. The following areexemplary of such additives and are commercially available from TheLubrizol Corporation: Lubrizol 677A, Lubrizol 1095, Lubrizol 1097,Lubrizol 1360, Lubrizol 1395, Lubrizol 5139, and Lubrizol 5604, amongothers; from Ciba Corporation: Irgalube® 62, Irgalube 211, Irgalube 232,Irgalube 349, Irgalube 353, Irgalube TPPT, Irgafos® OPH, among others;and from Chemtura Corporation: Weston® 600, Weston DLP, Weston TPP,among others.

Examples of friction modifiers include fatty acid esters and amides,organo molybdenum compounds, molybdenum dialkyldithiocarbamates,molybdenum dialkyl dithiophosphates, molybdenum disulfide,tri-molybdenum cluster dialkyldithiocarbamates, non-sulfur molybdenumcompounds and the like. The following are exemplary of molybdenumadditives and are commercially available from R. T. Vanderbilt Company,Inc.: Molyvan A, Molyvan L, Molyvan 807, Molyvan 856B, Molyvan 822,Molyvan 855, among others. The following are also exemplary of suchadditives and are commercially available from Asahi Denka Kogyo K.K.:SAKURA-LUBE 100, SAKURA-LUBE 165, SAKURA-LUBE 300, SAKURA-LUBE 310G,SAKURA-LUBE 321, SAKURA-LUBE 474, SAKURA-LUBE 600, SAKURA-LUBE 700,among others. The following are also exemplary of such additives and arecommercially available from Akzo Nobel Chemicals GmbH: Ketjen-Ox 77M,Ketjen-Ox 77TS, among others. Naugalube MolyFM is also exemplary of suchadditives and is commercially available from Chemtura Corporation.

Examples of VI improvers include olefin copolymers and dispersant olefincopolymers, and the like. An example of a pour point depressant ispolymethacrylate, and the like. An example of an antifoamant ispolysiloxane, and the like. Examples of rust inhibitors arepolyoxyalkylene polyol, benzotriazole derivatives, and the like.Examples of metal deactivators include triazole, benzotriazole,2-mercaptobenzothiazole, 2,5-dimercaptothiadiazole, tolyltriazolederivatives, N,N′-disalicylidene-1,2-diaminopropane, and the like. Thefollowing are exemplary of metal deactivators and are commerciallyavailable from Ciba Corporation: Irgamet® 30, Irgamet 39, and Irgamet42.

Compositions, when they contain these additives, are typically blendedinto the base oil in amounts such that the additives therein areeffective to provide their normal attendant functions. Representativeeffective amounts of such additives are illustrated in Table 2.

TABLE 2 Preferred Additives Weight % More Preferred Weight % V.I.Improver    1-12  1-4 Corrosion Inhibitor 0.01-3 0.01-1.5 Antioxidant0.01-5 0.01-1.5 Dispersant  0.1-10 0.1-5  Lube Oil Flow Improver 0.01-20.01-1.5 Detergent/Rust Inhibitor 0.01-6 0.01-3   Pour Point Depressant  0.01-1.5 0.01-0.5 Anti-foaming Agents  0.001-0.1 0.001-0.01 Anti-wearAgents 0.001-5  0.001-1.5  Seal Swell Agents  0.1-8 0.1-4  FrictionModifiers 0.01-3 0.01-1.5 Lubricating Base Oil Balance Balance

When other additives are employed, it may be desirable, although notnecessary, to prepare additive concentrates comprising concentratedsolutions or dispersions of the subject additives of this invention (inconcentrate amounts hereinabove described), together with one or more ofsaid other additives (said concentrate when constituting an additivemixture being referred to herein as an additive-package) whereby severaladditives can be added simultaneously to the base oil to form thelubricating oil composition. Dissolution of the additive concentrateinto the lubricating oil can be facilitated by solvents and by mixingaccompanied by mild heating, but this action is not essential. Theconcentrate or additive-package will typically be formulated to containthe additives in proper amounts to provide the desired concentration inthe final formulation when the additive-package is combined with apredetermined amount of base lubricant. Thus, the subject additives ofthe present invention can be added to small amounts of base oil or othercompatible solvents along with other desirable additives to formadditive-packages containing active ingredients in collective amountsof, typically, from about 2.5 to about 90 percent, preferably from about15 to about 75 percent, and more preferably from about 25 percent toabout 60 percent by weight additives in the appropriate proportions withthe remainder being base oil. The final formulations can typicallyemploy about 1 to 20 weight percent of the additive-package with theremainder being base oil.

All of the weight percentages expressed herein (unless otherwiseindicated) are based on the active ingredient (AI) content of theadditive, and/or upon the total weight of any additive-package, orformulation, which will be the sum of the AI weight of each additiveplus the weight of total oil or diluent.

In general, the additives of the present invention are useful in avariety of lubricating oil base stocks. The lubricating oil base stockis any natural or synthetic lubricating oil base stock fraction having akinematic viscosity at 100° C. of about 2 to about 200 cSt, morepreferably about 3 to about 150 cSt, and most preferably about 3 toabout 100 cSt.

The lubricating oil base stock can be derived from natural lubricatingoils, synthetic lubricating oils, or mixtures thereof. Suitablelubricating oil base stocks include base stocks obtained byisomerization of synthetic wax and wax, as well as hydrocracked basestocks produced by hydrocracking (rather than solvent extracting) thearomatic and polar components of the crude. Natural lubricating oilsinclude animal oils, such as lard oil, tallow oil, vegetable oils (e.g.,canola oils, castor oils, sunflower oils), petroleum oils, mineral oils,and oils derived from coal or shale.

Synthetic oils include hydrocarbon oils and halo-substituted hydrocarbonoils, such as polymerized and interpolymerized olefins, gas-to-liquidsprepared by Fischer-Tropsch technology, alkylbenzenes, polyphenyls,alkylated diphenyl ethers, alkylated diphenyl sulfides, as well as theirderivatives, analogs, homologs, and the like. Synthetic lubricating oilsalso include alkylene oxide polymers, interpolymers, copolymers, andderivatives thereof, wherein the terminal hydroxyl groups have beenmodified by esterification, etherification, etc. Another suitable classof synthetic lubricating oils comprises the esters of dicarboxylic acidswith a variety of alcohols. Esters useful as synthetic oils also includethose made from C₅ to C₁₈ monocarboxylic acids and polyols and polyolethers. Other esters useful as synthetic oils include those made fromcopolymers of α-olefins and dicarboxylic acids which are esterified withshort or medium chain length alcohols.

Silicon-based oils, such as the polyalkyl-, polyaryl-, polyalkoxy-, orpolyaryloxy-siloxane oils and silicate oils, comprise another usefulclass of synthetic lubricating oils. Other synthetic lubricating oilsinclude liquid esters of phosphorus-containing acids, polymerictetrahydrofurans, poly α-olefins, and the like.

The lubricating oil may be derived from unrefined, refined, re-refinedoils, or mixtures thereof. Unrefined oils are obtained directly from anatural source or synthetic source (e.g., coal, shale, or tar andbitumen) without further purification or treatment. Examples ofunrefined oils include a shale oil obtained directly from a retortingoperation, a petroleum oil obtained directly from distillation, or anester oil obtained directly from an esterification process, each ofwhich is then used without further treatment. Refined oils are similarto unrefined oils, except that refined oils have been treated in one ormore purification steps to improve one or more properties. Suitablepurification techniques include distillation, hydrotreating, dewaxing,solvent extraction, acid or base extraction, filtration, percolation,and the like, all of which are well-known to those skilled in the art.Re-refined oils are obtained by treating refined oils in processessimilar to those used to obtain the refined oils. These re-refined oilsare also known as reclaimed or reprocessed oils and often areadditionally processed by techniques for removal of spent additives andoil breakdown products.

Lubricating oil base stocks derived from the hydroisomerization of waxmay also be used, either alone or in combination with the aforesaidnatural and/or synthetic base stocks. Such wax isomerate oil is producedby the hydroisomerization of natural or synthetic waxes or mixturesthereof over a hydroisomerization catalyst. Natural waxes are typicallythe slack waxes recovered by the solvent dewaxing of mineral oils;synthetic waxes are typically the wax produced by the Fischer-Tropschprocess. The resulting isomerate product is typically subjected tosolvent dewaxing and fractionation to recover various fractions having aspecific viscosity range. Wax isomerate is also characterized bypossessing very high viscosity indices, generally having a VI of atleast 130, preferably at least 135 or higher and, following dewaxing, apour point of about −20° C. or lower.

The additives of the present invention are especially useful ascomponents in many different lubricating oil compositions. The additivescan be included in a variety of oils with lubricating viscosity,including natural and synthetic lubricating oils and mixtures thereof.The additives can be included in crankcase lubricating oils forspark-ignited and compression-ignited internal combustion engines. Thecompositions can also be used in gas engine lubricants, steam and gasturbine lubricants, automatic transmission fluids, gear lubricants,compressor lubricants, metal-working lubricants, hydraulic fluids, andother lubricating oil and grease compositions. The additives can also beused to stabilize motor fuel compositions.

The advantages and the important features of the present invention willbe demonstrated in the following examples.

EXAMPLES Example 1

Table 3 lists the chemical properties of the antioxidants selected fortesting. Depending on the test program and lubricant formulation, otheradditives such as antiwear agent, detergents, dispersant, pour pointdepressant, viscosity index improver, metal deactivator, corrosioninhibitor etc. were also used.

TABLE 3 Chemical names and descriptions to the antioxidants selected inthis study Identification Chemical name Elemental Durad AX182,2′-thiobis(4-methyl-6-tert-butyl- 8.5 wt % S phenol) Naugalube-438LNonylated diphenylamine 3.5 wt % N Naugalube-640 Butylated, octylateddiphenylamine 4.2 wt % N Naugalube-5313,5-di-t-butyl-4-hydroxyhydrocinnamic — acid, C₇-C₉ branched alkyl esterThe base oils selected for this study included a solvent refined APIGroup I base stock, a hydrocracked API Groups II and III base stockobtained from different refineries in the U.S.

Blends were made by adding additives to the specific base oils andmechanically mixing at 65° C. for approximately 15 minutes under theprotection of nitrogen. All blends were freshly prepared prior to theaccelerated oxidation bench tests employing a pressurized differentialscanning calorimetry (PDSC) and a Rotating Pressure Vessel OxidationTest (RPVOT) apparatus.

The PDSC examines an oil's oxidative stability under thin-film oxidationconditions. In the isothermal mode where PDSC temperature is maintainedat a predetermined value, a test oil's oxidation stability is rankedaccording to the oxidation induction time (OIT), corresponding to anexothermic release of heat caused by the onset of oxidation of the oil.Oil giving longer OIT is generally considered more resistant tooxidation. To expedite the PDSC testing process, each test oil waspre-treated with 50 ppm of oil soluble iron derived from ferricnaphthenate. Each blend was tested in duplicate using the followinginstrumental conditions and the average OIT was determined. In order toobtain reproducible results, the optimum PDSC test temperatures weredetermined to be 185° C. for the Group I oil blends and 200° C. for allother blends utilizing the Groups II and III base stocks.

Isothermal temperature: 185° C. for Group I oil blends; 200° C. forGroup II, III oil blends

Temperature ramp: 40° C./min

Pressures: 500 psi

O₂ flow: 100 ml/min

Sample size: ˜1.5 mg

Pan: Aluminum, open

Catalyst: 50 ppm of ferric naphthenate (8% in mineral oil)

All RPVOT tests were performed in accordance with the ASTM D2272standard test procedures and the results (OIT) from the average ofduplicate runs were reported. An oil giving longer oxidation inductiontime is generally considered more resistant to oxidation.

Autosynergisin of lubricant antioxidants is a type of synergisticresponse resulting from two different functions in the same molecule.Like the Durad AX18, antioxidants with functional groups that provideradical scavenging and hydroperoxide decomposing functions may exhibitautosynergy under proper oxidation conditions, thus leading to superiorperformances. In the first round of the bench tests, Durad AX18 wascompared to the Naugalube 531 under an equal level (1.0 wt %) in aseries of fully formulated passenger car motor oils (PCMO) andindustrial R&O turbine oils, each utilizing an API Group I, II, or IIIbase oil. The engine oil blends were tested in the PDSC while theturbine oils were tested in the RPVOT with the test conditions describedabove. Tables 4 and 5 show the PDSC and the RPVOT test results,respectively, for all blends.

TABLE 4 PDSC results for Durad AX18 and Naugalube-531, tested at 1.0 wt% in PCMO formulations utilizing Group I, II or III base stock Base OilPDSC PDSC OIT, PDSC OIT, Mean PDSC, AO Type T (° C.) run 1 (min) run 2(min) (min) AX18 Group I 185 22.02 22.28 22.15 NL-531 Group I 185 4.654.48 4.75 AX18 Group II 200 9.05 10.42 9.74 NL-531 Group II 200 <1 <1 <1AX18 Group III 200 10.91 10.72 10.82 NL-531 Group III 200 <1 <1 <1

TABLE 5 RPVOT results for Durad AX18 and Naugalube-531, tested at 1.0 wt% in Industrial turbine oil formulations utilizing Group I, II or IIIbase stock AO Base Oil Type RPVOT OIT (min) AX18 Group I 251 NL-531Group I 264 AX18 Group II 416 NL-531 Group II 200 AX18 Group III 488NL-531 Group III 300

The AX18 significantly outperformed the 531 in most formulations exceptfor the Group I oil based turbine oil formulation. The loss of responsein this oil was most likely due to the chemical nature of the base stockand the test method (the RPVOT) employed. Traces of nitrogen, sulfur,and oxygen-containing heterocycles, together with mercaptans (RSH),thioethers (R—S—R), and disulfides (R—S—S—R) are known to be an integralpart of the complex composition of the Group I base oils. If the ratioof aromatics and sulfur is kept at an optimum level, the natural oilresistance to oxidation is maximized. It is therefore possible that dueto the presence of these naturally occurred species the Naugalube-531containing Group I turbine oil outperformed the analogous Group II oilin the RPVOT. It is also possible that because of these naturallybalanced antioxidants the performance benefit derived from the furtheraddition of sulfur was limited in the Group I oil. By introducingexcessive sulfur, not only will the optimum sulfur/aromatic balance ofthe Group I oil be altered, the sulfur may also undergo hydrolysis inthe RPVOT test environment to form sulfur acid species which are knownto promote oxidation. Since the severely hydrocracked Groups II and IIIoils contain no appreciable sulfur, the sulfur derived from the AX18 hada strong and positive effect on the overall autosynergism of theantioxidant.

Having demonstrated the effect of base oil sulfur on the performance ofAX18, the antioxidant was further examined in conjunction with theNaugalube-438L in the Groups II and III oils based lubricantformulations. A total of 1.0 wt % of varying combinations of the twoantioxidants was blended along with other additives to form passengercar motor oils and industrial R&O turbine oils that were tested in thePDSC (@200° C.) and the RPVOT, respectively. The test results obtainedare given in Tables 6 and 7.

TABLE 6 PDSC results for Durad AX18 in varying combinations withNaugalube-438L, tested in PCMO formulations utilizing Group II or IIIbase stock. AX18, NL-438L, PDSC OIT, PDSC OIT, wt % NL-531, wt % BaseOil Type run 1 (min) run 2 (min) Mean PDSC, 1.0 Group II 21.66 22.0521.86 0.5 0.5 Group II 20.56 20.52 20.54 0.25 0.75 Group II 23.34 24.7424.04 0.25 0.75 Group II 20.36 18.98 19.67 1.0 Group III 21.22 21.3421.28 0.25 0.75 Group III 24.83 25.14 24.99 0.25 0.75 Group III 20.4521.47 20.96

TABLE 7 RPVOT results for Durad AX18 in varying combinations withNaugalube-438L, tested in turbine oil formulations utilizing Group II orIII base stock. NL-531, NL-438L, AX18, wt % wt % wt % Base Oil TypeRPVOT OIT, min 1.0 Group II 1270 0.5 0.5 Group II 1162 0.25 0.75 GroupII 1344 0.25 0.75 Group II 1117 1.0 Group III 1352 0.5 0.5 Group III1253 0.25 0.75 Group III 1723 0.25 0.75 Group III 1388

As compared to the AX18 and Naugalube-531 data listed in Tables 4 and 5,the Naugalube-438L on its own outperformed the two phenolic antioxidantsby generating significantly longer oxidation induction times in bothPDSC and RPVOT. Upon mixing the Naugalube-438L with the AX18@3:1 ratio(w/w) and holding the total concentration of the antioxidants constantlyat 1.0 wt %, a unique synergistic response was observed from theresulting mixtures. This response is first evident in the longer PDSCinduction times (24.04; 24.99 minutes) of the mixtures over thecorresponding 438L-only blends for the two groups of PCMO (Table 6). Atthis mixing ratio, the RPVOT performance of the turbine oils was alsoimproved considerably with the oxidation induction time being driven toabove 1300 minutes for the Group II oil and over 1700 minutes for theGroup III oil (Table 7). The non S-containing Naugalube-531, which wasincluded as reference in this round of tests, did not appear to besynergistic with the Naugalube-438L regardless of the formulation andtest method.

Reproduction of the Naugalube-438L/Durad AX18 synergism by using anothertype of alkylated diphenylamine antioxidant, the Naugalube-640, wassuccessful as well. As shown in Tables 8 and 9, considerable levels ofsynergism were observed for the Naugalube-640 and AX18 mixtures when thetwo antioxidants were blended at the 3:1 (w/w) ratio.

TABLE 8 PDSC results for Durad AX18 in varying combinations withNaugalube-640, tested in PCMO formulations utilizing Group II or IIIbase stock. AX18, NL-531, NL-640, PDSC OIT, PDSC OIT, Mean PDSC, wt % wt% wt % Base Oil Type run 1 (min) run 2 (min) (min) 1.0 Group II 21.5422.67 22.11 0.5 0.5 Group II 21.98 21.47 21.73 0.25 0.75 Group II 25.2625.62 25.44 0.25 0.75 Group II 20.92 21.81 21.37 1.0 Group III 22.6423.7 23.17 0.25 0.75 Group III 25.77 25.33 25.55 0.25 0.75 Group III21.33 21.21 21.27

TABLE 9 RPVOT results for Durad AX18 in varying combinations withNaugalube-640, tested in turbine oil formulations utilizing Group II orIII base stock AX18, NL-531, wt % wt % NL-640, wt % Base Oil Type RPVOTOIT, min 1.0 Group II 1744 0.5 0.5 Group II 1650 0.25 0.75 Group II 18630.25 0.75 Group II 1554 1.0 Group III 1779 0.5 0.5 Group III 1353 0.250.75 Group III 1860 0.25 0.75 Group III 1665

The synergistic effects obtained from the AX18/ADPA mixtures canprobably be attributed to a combined response from the autosynergism ofthe AX18 itself and the interaction of its active hydroxyl group withthe Naugalube-438L through the homosynergistic mechanism that reinforceeach other. It is this kind of mixed synergism that provided theAX18/ADPA systems additional stabilization power in both PDSC and RPVOT.The detection and magnitude of this type of synergy appear to bedependent on the type of base oil, more specifically the sulfur andaromatic contents of the oil as explained earlier, and also the mixingratio of the two antioxidants involved. The importance of keeping theratio optimum is exemplified by the inferior performances of the 1:1(w/w) ratio of the AX18 and an ADPA in both PCMO and the turbine oils(Tables 4 through 7).

Example 2

Table 10 shows the relative oxidation stability results obtained byadding certain aminic and phenolic antioxidant blends to Group I andGroup III base stocks.

TABLE 10 Relative Oxidation Stability Results in Group I and Group IIBase Stocks (Minutes) Antioxidants Base Stock Aminic Phenolic Group IGroup II — — 25 25 — Durad AX15 107 278 — Durad AX16 137 623 — DuradAX32 120 270 Durad AX55 — 300 741 Durad AX57 — 320 1140 Durad AX15 is2,2′-thiodiethylene bis [3-(3,5-di-b-butyl-4-hydroxyphenyl) propionate.Durad AX16 is 4,4′-thio bis(2-t-butyl-5-methyl phenol). Durad AX32 istetrakismethylene (3,5-di-t-butyl-4-hydroxyhydrocinnamate) methane.Durad AX55 is octylated/styrenated diphenylamine Durad AX57 isbutylated/octylated diphenylamine

The five antioxidants are present at a 0.5% treat rate.

The relative oxidation stability of the various lubricant base stocks isdetermined by the rotating pressure vessel oxidation test (RPVOT), ASTMtest method number D 2272. The oxidation stability is assessed as thelifetime, which is recorded in minutes.

Oxidation stability of these types of formulations can also bedetermined by differential thermal analysis (DTA), a variation ofdifferential scanning calorimetry (DSC), ASTM test method number D 5483.The oxidation stability is assessed as the lifetime, which is recordedin minutes.

Table 11 shows examples of the relative oxidation stability resultsobtained by adding certain aminic and phenolic antioxidant blends toGroup I and Group III base stocks.

TABLE 11 RPVOT Lifetime for Antioxidant Blends (Minutes) AntioxidantBase Stock Exam- Group I Group III ple Aminic Phenolic Expected ActualExpected Actual — — — 25 — 25 1 Durad Durad 261 — 649 355 AX55 AX15 2Durad Durad 264 271 647 865 AX55 AX32 3 Durad Durad 277 255 968 385 AX57AX15 4 Durad Durad 283 300 1037 2580 AX57 AX16 5 Durad Durad 280 328 966730 AX57 AX32

The five antioxidant blends shown in Table 11 are present at a 0.5%total treat rate and an aminic/phenolic antioxidant ratio of 4:1(wt/wt).

Both expected results, in terms of calculated “expected” results of theindividual components based on Table 10 results, as well as actualresults are shown.

The results in Table 11 above illustrate that a synergistic effect, interms of anti-oxidancy, was seen with Blend 2 (Durad AX55+Durad AX32)when added to a Group III lubricant base stock, 865 minutes actualversus 647 minutes expected, 35% higher than expected.

Unexpectedly, a notable synergistic effect, in terms of anti-oxidancy,was seen with Blend 4 (Durad AX57+Durad AX16) when added to a Group IIIbase stock, 2580 minutes actual versus 1037 minutes expected, 150%higher than expected.

The only other formulation where a synergistic effect could be noted wasseen with Blend 5 (Durad AX57+Durad AX32) when added to a Group Ilubricant base stock, 328 minutes actual versus 280 minutes expected,17% higher than expected.

In all other blend examples shown in Table 11, the anti-oxidancyperformance in Group I and/or Group III base stock ranged from 60% lessthan expected (Blend 3 in Group III base stock) to 6% greater thanexpected (Blend 4 in Group I base stock).

Example 3

Table 12 shows the relative oxidation stability results obtained byadding certain aminic and phenolic anti-oxidant blends to industrialturbine oils utilizing a Group II or a Group III base stock. With thetype of antioxidant varied in the final formulations, all otheradditives, including a metal deactivator, a corrosion inhibitor, and adefoamer in the turbine oils remained the same and were kept at aconstant level.

TABLE 12 Relative Oxidation Stability Results (Minutes) for Turbine OilsUtilizing Group II and Group III Base Stocks Base Anti-Oxidants StockEmployed Aminic Phenolic Group II Group III — — — Durad AX18 416 488 —Naugalube 531 200 300 Naugalube 438L — 1270 1352 Naugalube 640 — 17441779 Durad AX18 is 2,2-thiobis(6-t-butyl-4-methylphenol) Naugalube 531is 3,5-di-t-butyl-4-hydroxyhydrocinnamic acid, C₇-C₉ branched alkylester Naugalube 438L is nonylated diphenylamine Naugalube 640 isbutylated-, octylated diphenylamine

The four antioxidants are present at a 1.0 wt % treat rate. The relativeoxidation stability of the various lubricant base stocks is determinedby RPVOT, ASTM test method number D 2272. The oxidation stability isassessed as the lifetime, which is recorded in minutes.

Table 13 shows examples of the relative oxidation stability resultsobtained by adding certain aminic and phenolic anti-oxidant blends toindustrial turbine oils utilizing Group II and Group III base stocks.

TABLE 13 RPVOT Lifetime for Anti-Oxidant Blends (Minutes) Base StockEmployed Anti-Oxidant Group II Group III Blend ID Aminic PhenolicExpected Actual Expected Actual 6 Naugalube 438L Naugalube 531 1003 11171089 1338 7 Naugalube 640 Naugalube 531 1358 1554 1409 1665 8 Naugalube438L Durad AX18 1057 1334 1136 1723 9 Naugalube 640 Durad AX18 1412 18631456 1860

The four anti-oxidant blends shown in Table 13 are present at a 1.0 wt %total treat rate and an aminic/phenolic anti-oxidant ratio of 3:1(wt/wt). Both expected results, in terms of calculated “expected”results of the individual components, based on Table 5 results, as wellas actual results are shown.

The results in Table 13 above illustrate that a synergistic effect, interms of anti-oxidancy, was seen with blend 6 (Naugalube 438L+Naugalube531) utilizing a Group II or Group III base stock. The actual minuteswere 1117 versus the expected 1003 minutes for the Group II oil, being11% higher. While for the Group III oil, 1338 minutes actual versus 1089minutes expected, being 23% higher.

Similar levels of synergistic effect, in terms of anti-oxidancy, wereseen with blend 7 (Naugalube 640+Naugalube 531) utilizing a Group II orGroup III base stock. The actual minutes were 1554 versus the expected1358 minutes for the Group II oil, being 14% higher. While for the GroupIII oil, 1665 minutes actual versus 1409 minutes expected, being 18%higher.

Unexpectedly a notable synergistic effect, in terms of anti-oxidancy,was seen with blend 8 (Naugalube 438L+Durad AX18) utilizing a Group IIor Group III base stock. The actual minutes were 1334 versus theexpected 1057 minutes for the Group II oil, being 26% higher. While forthe Group III oil, 1723 minutes actual versus 1136 minutes expected,being 52% higher.

Similar levels of synergistic effect, in terms of anti-oxidancy, wereseen with blend 9 (Naugalube 640+Durad AX18) utilizing a Group II orGroup III base stock. The actual minutes were 1863 versus the expected1412 minutes for the Group II oil, being 32% higher. While for the GroupIII oil, 1860 minutes actual versus 1456 minutes expected, being 28%higher.

In view of the many changes and modifications that can be made withoutdeparting from principles underlying the present invention, referenceshould be made to the appended claims for an understanding of the scopeof the protection to be afforded the invention.

1. A lubricant composition comprising: (A) at least one lubricating oilselected from the group consisting of natural and synthetic lubricatingbase oils; (B) 0.1 to 5.0 weight percent based on the weight of thecomposition of at least one diphenylamine of the formula:

 wherein R₁ and R₂ are independently selected from the group consistingof hydrogen, alkyl of 1 to 12 carbon atoms, styryl, and α-alkyl styryl,provided that at least one of R₁ and R₂ is not hydrogen; and (C)2,2′-thiobis(4-methyl-6-tert-butyl-phenol) wherein the weight ratio ofthe diphenylamine to 2,2′-thiobis(4-methyl-6-tert-butyl-phenol) isgreater than 3:1.
 2. The composition of claim 1 which also comprises anadditional component selected from the group consisting of triarylphosphate esters, tri-alkyl phosphate esters, alkyl mono-acidephosphates, alkyl di-Acid phosphates, alkyl phosphites, aryl phosphites,liquid phenolic antioxidants, aminic antioxidants, carboxylate esters,aromatic hydrocarbons, and liquid glycols.
 3. A method of increasing theoxidation stability of a lubricant comprising adding thereto (A) 0.1 to5.0 weight percent based on the weight of the composition of at leastone diphenylamine of the formula:

 wherein R₁ and R₂ are independently selected from the group consistingof hydrogen, alkyl of 1 to 12 carbon atoms, styryl, and α-alkyl styryl,provided that at least one of R₁ and R₂ is not hydrogen; and (B)2,2′-thiobis(4-methyl-6-tert-butyl-phenol) wherein the weight ratio ofthe diphenylamine to 2,2′-thiobis(4-methyl-6-tert-butyl-phenol) isgreater than 3:1 and the lubricant is selected from the group consistingof natural and synthetic lubricating base oils.
 4. The composition ofclaim 1 wherein the weight ratio of the diphenylamine to2,2′-thiobis(4-methyl-6-tert-butyl-phenol) ranges from about 3:1 toabout 4:1.
 5. The method of claim 3 wherein the weight ratio of thediphenylamine to 2,2-thiobis(4-methyl-6-tert-butyl-phenol) ranges fromabout 3:1 to about 4:1.
 6. The composition of claim 1 wherein R₁ and R₂are independently selected from the group consisting of hydrogen methyl,ethyl and isomers of propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyland decyl.
 7. The composition of claim 1 wherein the diphenylamine isselected from the group consisting of nonylated diphenylamine;butylated, octylated diphenylamine; and octylated, styrenateddiphenylamine.
 8. The composition of claim 1 wherein the lubricating oilis selected from the group consisting of Group I, Group II and Group IIIAPI Base Oil Category base stocks.
 9. The method of claim 3 wherein R₁and R₂ are independently selected from the group consisting of hydrogenmethyl, ethyl and isomers of propyl, butyl, pentyl, hexyl, heptyl,octyl, nonyl and decyl.
 10. The method of claim 3 wherein thediphenylamine is selected from the group consisting of nonylateddiphenylamine; butylated, octylated diphenylamine; and octylated,styrenated diphenylamine.
 11. The method of claim 3 wherein thelubricant is selected from the group consisting of Group I, Group II andGroup III API Base Oil Category base stocks.